Effects of Valsalva manoeuvres and the 'CO₂-off' effect on cerebral blood flow

Previous research has shown that a rapid drop in inhaled carbon dioxide (CO₂) partial pressure reduces cerebral blood flow and may induce faintness - the 'CO₂-off' effect. The aims of this study were to investigate the effects of performing Valsalva manoeuvres while experiencing the 'CO₂-off' effect and whether symptoms occur that are sufficient to jeopardise submarine tower escape.

Twenty male volunteers, mean (SD) age 34.7 (8.5) years each completed three tests. The first test was to perform Valsalva manoeuvres breathing air. The second and third tests involved breathing a high CO₂ mix (5% CO₂/16% O(2)/79% N₂) for 1 h prior to switching to breathe O₂ and performing Valsalva manoeuvres, or switching to breathe air for 1 min then O₂ and performing Valsalva manoeuvres. Blood pressure, cerebral blood flow velocity, electrocardiogram, and respiration were monitored throughout. A subjective questionnaire was administered at intervals to monitor symptom type and severity.

Valsalva manoeuvres breathing air resulted in a 31% reduction in cerebral blood flow. Breathing high CO₂ caused a sustained increase in cerebral blood flow and symptoms of breathlessness and headache. Following the gas switch from high CO₂, some subjects reported faintness, headache and nausea. Cerebral blood flow dropped by 34% when switching from breathing high CO₂ to O₂, by 35% when switching to air then by a further 3% when switching from air to O₂. In both circumstances there was a further drop of 14% after performing the Valsalva manoeuvres. The drop in cerebral blood flow in subjects that reported faintness was greater than that in the subjects who did not, but this difference was not significant.

Transient faintness or headache may occur in the escape tower during pressurisation, but this should be shortlived and not incapacitating.